Question

I was just digging around in the commons-io library and found this:

Keeps track of files awaiting deletion, and deletes them when
an associated marker object is reclaimed by the garbage collector.

This can be found in the documentation for the FileCleaningTracker object.

Now I am just curious how I can do this by myself? How can my code detect when an object is reclaimed by the garbage collector?

Answer 1

According to the source code, it uses the PhantomReference class. According to the documentation:

Phantom reference objects, which are enqueued after the collector determines that their referents may otherwise be reclaimed. Phantom references are most often used for scheduling pre-mortem cleanup actions in a more flexible way than is possible with the Java finalization mechanism.

If the garbage collector determines at a certain point in time that the referent of a phantom reference is phantom reachable, then at that time or at some later time it will enqueue the reference.

In order to ensure that a reclaimable object remains so, the referent of a phantom reference may not be retrieved: The get method of a phantom reference always returns null.

Unlike soft and weak references, phantom references are not automatically cleared by the garbage collector as they are enqueued. An object that is reachable via phantom references will remain so until all such references are cleared or themselves become unreachable.

The PhantomReference constructor accepts two arguments:

referent – the object the new phantom reference will refer to

q – the queue with which the reference is to be registered, or null if registration is not required

The q argument is an instance of the ReferenceQueue class. The PhantomReference will be added to this ReferenceQueue when it’s referent becomes phantom reachable. When this happens, you can retrieve the PhantomReference by using the poll() or remove() methods of the ReferenceQueue class.

For example:

T objectToWatch = ...;
ReferenceQueue<T> referenceQueue = new ReferenceQueue<T>();
new PhantomReference<T>(objectToWatch, referenceQueue);

// Later on, probably in another thread...
Reference<? extends T> nextReference = referenceQueue.remove();
// Tidy up!

Note: PhantomReference has sibling classes named SoftReference and WeakReference that may also be of use. The relationship between these are documented in the java.lang.ref package documentation.